One of the major problems in the design of power electronic circuits (e.g., motor drives and amplifiers) is the protection of the semiconductor elements in such circuits from overheating. A semiconductor element such as a transistor is not 100% efficient in utilizing electrical energy. As a result, some of the electrical energy in the semiconductor element is converted into thermal energy. The generation of thermal energy results in a rise in the temperature of the semiconductor element. As is well known in the art, the performance of a semiconductor element deteriorates with temperature. In addition, if the temperature of the semiconductor is above a certain value, the semiconductor element could be permanently destroyed.
One way to remove the thermal energy from a semiconductor element, thereby reducing its temperature, is by mounting the semiconductor element to a heat sink. Typically, a heat sink has a large surface area. The effect of mounting the semiconductor element to the heat sink is to increase the surface area of the element. Since the rate of heat transfer from a heat source to the ambient is proportional to the surface area of the heat source, the mounting of the semiconductor element to the heat sink allows faster heat transfer to the ambient. As a result, the temperature of the semiconductor element can be kept at a lower temperature. Examples of prior art heat sinks are described in U.S. Pat. No. 4,879,633 issued to Kaufman and U.S. Pat. No. 4,965,660 issued to Ogihara, et al.
Another major problem in the design of a power electronic circuit is the generation of electromagnetic interference (emi) by the circuit. This problem frequently arises when current is switched at a high frequency by an active device, e.g., a semiconductor transistor switching element, in the circuit. There are two major forms of emi. The first form is interference resulting from electromagnetic waves generated by the circuit and radiated to the ambient. The second form is interference resulting from electromagnetic noise that is propagated along circuit wirings.
The first form of emi arises because the wirings connecting the circuit elements in the circuit act as antennas which convert the current in the circuit into electromagnetic waves. These electromagnetic waves are radiated to the air and have potential for interfering with other electromagnetic waves carrying desirable signals, or nearby circuits sensitive to electromagnetic radiation.
The second form of emi arises because the wiring connecting the circuit elements in the circuit could act as transmission lines. Electrical energy generated at one part of the circuit can propagate through the transmission lines to a second part of the circuit even though these two parts are not connected. This electrical energy would appear as noise at the second part of the circuit. If the noise reaches the power supply of the circuit and degrades the regulation of the power supply, the operation of the whole circuit could be affected. In addition, the noise could also propagate outside the circuit to the power line through the power supply. In this case, other electronic devices sharing the same power line with the circuit would also suffer from the effects of the noise generated by the circuit.
Since the electromagnetic interference could affect a large number of electronic apparatus, the governments of many countries have set limits to the amount of electromagnetic interference which can be emitted by an electronic apparatus. If the amount of electromagnetic interference emitted by an electronic apparatus exceeds the limit, the apparatus may be prohibited from being used or sold. Consequently, the reduction of electromagnetic interference becomes an important part of the design of an electronic apparatus.